Atomic boron halides

BCI3 + 3H2O -> 3HC1 + H3BO3 As there are only three pairs of shared electrons in the outer shell of the boron atom, boron halides form very stable addition compounds with ammonia by the acceptance of a lone electron pair in a coordinate bond to complete a shared octet. 

The observed order of reactivity for the boron halides is BF3 < BCI3 < BB F3 < BI3 From electronegativity considerations, we might expect the opposite trend. The electronegativity difference between boron ( = 2.0) and fluorine ( () = 4.0) is 2, whereas boron and iodine ()Y — 2.5) differ by only 0.5. Thus, fluorine atoms... 

In reactions between Lewis acids and bases such as amines and boranes or boron halides, bulky substituents on one or both species can affect the stability of the acid-base adduct. Perhaps the most straightforward type of effect is simple steric hindrance between substituents on the nitrogen atom and similar large substituents on the boron atom. Figure 9.3 is a diagrammatic sketch of the adduct between molecules of tripropylamine and triethylborane. This phenomenon is known as front or F-... 

The halogen atom on organic halides may undergo exchange reactions with halide ions on boron halides ... 

Among the boron halides, B4CI4 is of interest as both B4H4 and B411 do not exist. Figure 13.3.7 shows the molecular structure of B4CI4, which has a tetrahedral B4 core consolidated by four terminal B-Cl 2c-2e bonds and four BBB 3c-2e bonds on four faces. This structure is further stabilized by a-n interactions between the lonepairs of Cl atoms and BBB 3c-2< bonds. The... 

Diorgano tellurium dihalides form complexes with iodine and interhalogen compounds organic compounds with N, P, O, S, and Se donor atoms boron, aluminum, and gallium trihalides antimony pentachloride and mercury(II) halides. 

Boron halides, such as BF3 or BC13, are electron-deficient molecules because they do not have an octet of electrons surrounding the boron atom. In accord with this property, they tend to act as strong Lewis acids by accepting electron pairs from bases to form stable acid-base adducts. Such electron donors as pyridine or ether can be used ... 

Salts containing these ions can be isolated as solids with large cations such as R4P+. Because they have an unshared pair of electrons on the Sn atom, these ions are Lewis bases that form adducts with Lewis acids such as boron halides ... 

Alkaline hydrogen peroxide easily oxidizes practically all alkyl- and cycloalkyl-boranes in a rapid and quantitative fashion. There is a reactivity trend of R3B > R2BX > RBX2 (X = halogen, OH, OR note that boron halides will anyway be hydrolyzed to hydroxides under the oxidation conditions), which is consistent with reduced acceptor ability of the boron atom when an electron pair of an adjacent group interacts with the vacant boron oibital. Increasing die steric hindrance around the boron atom may inhibit the reaction to the point at which it ceases altogether. - ... 

Thus, the fractionation of boron isotopes between boron trifluoride and its molecular addition compounds may be explained in terms of unique characteristics of the boron and fluorine atoms. The model presented here adequately describes the direction of enrichment as well as the magnitude of the equilibrium constant. It accounts for observed variations in the size of fractionation factor for different donors as well as for different substituents on the same donor. The model correctly predicts the isotopic behavior of other boron halides when these are substituted for BF3 in the exchange reaction. Finally, the proposed model provides insight into the design of a practical chemical exchange system for the separation of boron isotopes. 

Boron Halides.—An ab initio MO calculation on BF2, using a near-Hartree-Fock atomic basis, predicts a bond angle of 120° and a bond length of 1.22 A for the X2(Aj) ground state.250... 

Both boron and aluminium halides are Lewis acids but only aluminium halides exist as dimer whereas boron halides exists as monomers. This is due to the reasons that boron atom is so small that it cannot accommodate four large sized halides... 

---